This invention relates to a toner image transfer type electrographic copying apparatus designed to develope an electrostatic image carried on the surface of an electrostatic latent image carrier to change the image into a toner image and to transfer the toner image onto the surface of a sheet of copy paper.
In an electrographic copying apparatus of the type described, it is well known to provide the electrostatic latent image carrier, which may be, for example, a photosensitive member having a photoconductive layer on its surface, in the form of a belt A as shown in FIG. 1. The belt A or photosensitive body is stretched between two support rollers B and C, and means such as a charger D, an original image-illustrating lamp E, a SELFOC lens F, a developing device G, a transfer device H, a cleaning device 1a, a charge erasing lamp J, are provided around the belt A adjacent to the outer circumferential surface of the belt. With the start of a copying operation, as shown in FIG. 1, an image exposing unit integrally holding the charger D, the lamp E and the SELFOC lens F (a lens array formed by arranging light-transmitting optical fibers in a plurality of lines) is moved above a flat first image surface stretch a.sub.1 of the belt A positioned in an opposed relation with respect to an original K in the direction indicated by arrow P in FIG. 1. An electrostatic latent image corresponding to the original K is formed on the first surface stretch a.sub.1 by the unit travelling in the manner described. Thereafter, the belt A is rotated by rotation of support roller B and C in the direction M in the figure and the latent image formed on the first image surface stretch a.sub.1 is developed by a developing device G to provide a toner image. The toner image is thereafter transferred by the transfer device H onto the surface of a copy paper sheet N. Simultaneously therewith, as shown in FIG. 1, a second flat image surface stretch a.sub.2 on that side which is not opposed to the original K is moved to a position on the side opposing the original K between the support rollers B and C after the residual toner and charge have been removed from the stretch a.sub.2 by a cleaning device I and a charger erasing lamp J. When it is desired to make a plurality of copies in sequence, namely to obtain several copies sequentially from one original, the described copying step is repeated while an electrostatic latent image is sequentially formed on the first and second image surface stretches a.sub.1 and a.sub.2.
In the structure described, the developing device G and cleaning device I are positioned a large distance apart, i.e., so as to oppose each of curved portions a' and a" of belt A as shown in FIG. 1. Accordingly, when it is desired to reuse the toner collected by the cleaning device I to develop a latent image, the toner must be transferred by hand from the cleaning device I to the developing device G or a suitable automatic transfer mechanism utilized. The use of an automatic transfer mechanism, however, makes the copying apparatus large in size and complicated. To position the developing device G near the cleaning device I in an attempt to improve this situation, it is necessary to move the photosensitive belt A at least one rotation and a half for every copying operation, which reduces the copying speed particularly when continuous copying is desired.
On the other hand, in the structure described, image exposure is effected with respect to the flat first and second image surface stretches of the photosensitive belt. Accordingly, the structure described has the advantage that it not only produces no such distortion of an image as is seen in the exposure of the curved image surface of the circumference of a cylinder, but it also enables full-frame image exposure other than slit image exposure by use of the SELFOC lens. However, when the belt A is stopped, parts of the photosensitive belt corresponding to curved portions a' and a" formed along the outer circumference of each of the support rollers B and C between the image surface stretches a.sub.1 and a.sub.2 are wasted, i.e., the portions are not used for forming an electrostatic latent image. In addition thereto, the support rollers B and C have to be larger than a certain value in diameter from the viewpoint of the strength of curvature of the photosensitive belt A, which, in turn, results in an increase also in the length of the curved portions a' and a" over a certain length, with the result that the structure described makes it necessary to reduce the percentage of effective length the belt A.